1. Field of the Invention
The invention relates, generally, to three-phase electrical energy distribution systems and, in particular, electrical energy distribution systems furnished with current distribution bars. A particularly beneficial application of the invention relates to electrical energy distribution systems based on the use of current distribution bars provided in electrical energy distribution cabinets or cores of aircraft. However, the invention also applies to three-phase electrical energy distribution systems in all types of equipment, especially in batteries, converters, for example AC/DC, etc.
2. Description of the Relevant Art
As is known, an electrical energy distribution network within an aircraft has a pyramidal structure.
The electrical power is firstly produced by using part of the power provided by the engines of the aircraft to drive electrical energy generators. The electrical energy generated is provided to distribution cabinets, also referred to as distribution “cores”, so as thereafter to be redistributed either to loads, or to secondary distribution boxes, with different amperages. Three-phase electrical distribution bars, also known as “busbars”, are used to convey the currents within the distribution cabinet. The powers conveyed within the cabinet are relatively significant. They can reach values of the order of a megawatt.
The cross-section of the distribution bars determines the maximum current which can flow safely in the bars.
Each primary or secondary distribution cabinet integrates distribution components, the function of some of which is to switch the energy to a primary distribution bar, as is the case for the line contactors, to electrical loads aboard the aircraft or else to secondary distribution sub-networks which again redistribute the energy to loads of lesser power. The function of other distribution components is to protect the network in case especially of electrical faults, such as a short-circuit.
Systematically, each cable which exits a distribution cabinet in order to distribute the electrical energy is protected either by a breaker, or by a fuse, or by a contactor-breaker. The operation of these components is triggered on the basis of an overload of current. The triggering time is inversely proportional to the overload current.
The switching element furthest upstream, that is to say closest to the electrical source, consists of the line contactor. The protection furthest upstream of the electrical network is consequently that which commands the generator and controls the line contactor. This protection is based on a measurement of the current so as to identify an overload and to isolate the fault if no other downstream protection has been able to do so, consequently proving that the fault is situated at the level of the primary network, between the line contactor and the protection devices situated downstream. The protection provided at the level of the primary network can only be triggered after a sufficient duration so that the protections situated downstream can operate in the case where the fault were to be situated at their level. Today, this duration of triggering may be a maximum of 5 seconds, this being sufficiently long to cause damage.
Thus, the protection provided at the level of the generator and of the line contactor is the longest to be implemented. Therefore, a fault occurring directly downstream, on the primary network, could, having regard to the triggering time of this protection, cause damage before being isolated.
It will be noted moreover that the set of protection devices provided in primary or secondary distribution cabinets do not make it possible to ensure protection against all types of faults liable to be encountered in an electrical core.
Although they are effective for covering faults such as over-currents or short-circuits, certain types of short-circuits are however not covered by these protections.
For example, metallic objects (screwdrivers, nuts, etc.) left by error in an electrical cabinet during a maintenance operation, are liable to cause short-circuits when they come into contact with the electrical distribution bars. Such short-circuits will not be seen by the primary network protection devices and are liable to cause the occurrence of electric arcs liable to propagate along the distribution bars and to cause significant damage liable to compromise the safety of the aircraft.
The destructive effect of an arc occurs, however, only when the arc is slowed. In this case, it eats away the metal of the distribution bars, projecting molten metal around it. Such is also the case when it encounters an obstacle, be it metallic or insulating.
It has indeed been noted that an electric arc which propagates over distribution bars is generally of the order of 2 to 3 cm high, for values of current, frequency and voltage in the aeronautics sector. This is the reason why it is necessary to provide a safety zone of about 3 to 4 cm around the distribution bars in the distribution cabinets so as to prevent an electric arc which propagates on the bars from attaching onto a metallic element of the cabinet, which may turn out to be constraining.